Graphite is currently used as an anode material in lithium-ion batteries. The maximum theoretical capacity of a graphite anode is 372 mAh/g. In an attempt to improve the capacity of anodes, the researchers at Fujifilm Celltec Co. performed research on a new generation of lithium-ion cells employing amorphous tin-based composite oxide glasses as anode materials, which exhibited potentially large capacities (Y. Idota, A. Matsufuji, Y. Maekawa, and T. Miyasaki, Science, 276, 1395 (1997)). A number of research activities have been focused on tin-containing anode materials since then. However, despite all of such efforts, graphite is still the preferred material used in commercial lithium-ion batteries.
It is our understanding that the Fujifilm materials are essentially composites of various active tin oxides in other inactive oxides. According to earlier researches on the subject (see for example, I. A. Courtney and J. R. Dahn, J. Electrochem. Soc., 144, 2045 (1997); I. A. Courtney, W. R. McKinnon and J. R. Dahn, J. Electrochem. Soc., 146, 59 (1999)), when lithium electrochemically enters an anode formed from such materials during a first charge of a battery, the lithium reacts with oxygen in the tin oxide to form lithium oxide and the tin in the tin oxide becomes elemental tin nano-dispersed in situ in the framework of the lithium oxide. The lithium that reacts with the oxygen during the first charge, however, is lost and will not participate in any further electrochemical cycling within the practical voltage window of the battery. The consumed lithium results in an irreversible capacity loss for the battery. During subsequent cycling, the capacity of the battery is provided by the nano-dispersed tin that is alloyed and de-alloyed in an alloying process. The non-participating atoms in the glass (also called “spectator” atoms) provide the framework to absorb the large volume changes associated with the alloying process. Therefore, the more oxygen that is reacted with lithium in the material during the first charging cycle, the larger the irreversible capacity. The more inactive non-participation atoms (spectators) in the composite material, the better the cycle life. There is, however, a resulting lower reversible capacity.
For example, the earlier reported tin-containing glass materials typically exhibit more than 50% irreversible capacity, and have very poor cycle life unless the capacity is reduced to a level very similar to that of graphite by the addition of large amounts of inactive atoms in the oxide glass such as B2O5 and P2O5 clusters. Because of large irreversible capacity exhibited by such materials and poor structural stability, these materials are typically not used in commercial lithium ion cells.
In recent years, the focus of tin-based anode material research has shifted away from the oxide materials in favor of intermetallic alloy materials, such as Cu—Sn systems, Fe—Sn—C systems, Mo—Sn alloys, and the like. The intermetallic alloys, however, must be produced in oxygen free environments to control irreversible capacity losses. In addition, such materials are typically produced with high energy ball milling in an argon environment, which is expensive. The capacities of such materials are typically very similar to or even below those of graphite. The potential benefits of these materials are that a) the tin-based materials should be safer than graphite because the binding energy between tin and lithium is larger than that between graphite and lithium, and therefore the tin-based materials are less reactive with electrolytes during thermal abuses of the battery in the charged state; and b) the true density of the tin alloys are generally about twice of that of graphite and therefore the volumetric energy density of battery can be improved by employing such materials even if the specific capacity of the materials are the same as graphite.
Another suggested approach for forming anode materials includes reacting Li3N with SnO to obtain a composite of tin nano-dispersed in Li2O (D. L. Foster, J. Wolfenstine, J. R. Read, and W. K. Behl, Electrochem. Solid-state Lett. 3, 203 (2000)). However, because of the low reactivity between Li3N and SnO (the Li—N bond must be broken), it takes about 5 days of high energy ball milling for the reaction to occur, which is undesirable from a commercial processing standpoint.
Tin and silicon can each alloy with 4.4Li, and they each exhibit very large theoretical capacities of 990 mAh/g and 4200 mAh/g, respectively. Therefore, it is desirable to develop methods for incorporating such materials into electrodes for use with rechargeable batteries. It is also desirable to develop processes capable of producing tin and silicon containing compositions that may be used with electrodes.